I do not know of any RV which has city fill only. ALl have a Gravity fill though finding it may be a job and a half.... One RV it's under the hood in a locked box.. Look for a locked door about oh 5 or 6 inches square...
Now... you can fill from an external tank using a 12 volt pump.. There is a very common 12 volt water pump that is rated for drinking water (you have one in fact) it has 1/2 inch IPT inlet and outlet.. Adapt the 1/2 inch outlet to a garden hose.. put an inlet strainer on the inlet side and adapt to like 10' of clear "Vinyl" suction hose Stick the suction hose in your bucket and apply 12 volts.
What kind of pump is this.. why the water pump on your RV of course.. do not use it (though in some cases you can as it's already set up for it) use a 2nd one you get from Amazon or such.
There are also 12 volt and drill powered "Transfer" pumps. they usually have garden hose fittings but I'm not convinced they are potable water rated (Drinking) Rhe RV pump is.
